Tim,
I am pretty tall as well and my wife is smaller and shorter than your wife.
From my experience smaller, lighter people tend to slow down much more from heavy packs than bigger guys. Even though this seems to be more of an issue in ski mountaineering where packs are generally heavier.
However, I wouldn’t underestimate the importance of technique and efficiency, also on the uphill. You can “waste” a lot of energy trying to balance out bad kickturns, lifting your skis up by an inches with every step etc. This can really drain even fit guys very quickly.
You mentioned that this is only your second season, so I would probably start looking there.
1.7kg weight difference per foot is quite a lot as well.